As global inventory levels fall and oil supply aligns itself more closely to demand, the oil market’s sensitivity to any unexpected disruption is growing.

Brent has risen by 36 percent so far in the third quarter, and is set for its strongest performance between July and September since 2004. (For graphic, click reut.rs/2y3GcVX)

Against U.S. crude futures, it is trading at its largest premium in over two years, having doubled in just over a month. (For graphic, click reut.rs/2jVJRPw)

One driving factor has been the risk of a disruption to Kurdish flows amid tensions over the Kurdistan Regional Government’s decision to hold a referendum on independence, which took place on Monday.

Those risks have risen in the past two days after Iraq on Sunday urged countries to stop oil trade with its autonomous Kurdistan region in retaliation for the vote, and Turkish President Tayyip Erdogan threatened on Monday to cut off the Ceyhan pipeline that carries the Kurdish region’s entire oil output to the outside world.

The pipeline to Turkey’s port of Ceyhan usually pumps between 500,000-600,000 barrels per day.

That is equivalent to only around 0.5 percent of global daily production, but rising nervousness about potential disruptions helped send November Brent futures up by more than 2 percent on Monday to around $58 a barrel, its highest since July 2015.

That took Brent’s premium to U.S. crude futures to $6.50 a barrel, the biggest gap since August 2015.

Shipping sources said flows of Kurdish oil through the Ceyhan pipeline were normal on Monday, but Erdogan’s warning was clear. “We have the tap. The moment we close the tap, then it’s done,” he said in a speech at a forum in Istanbul.

Against Dubai crude futures, Brent is commanding its largest premium since early December 2016.

In theory, a larger premium makes Brent-linked crudes, such as those produced in the North Sea, West Africa, or in some parts of the Mediterranean, less attractive for buyers in say the United States or Asia.

But this divergence does not seem to have deterred consumers, who have absorbed a large overhang of unused crude in the last six weeks and pushed up prices for some physical barrels of oil to multi-month or even multi-year highs.